Vapor streams are today generated and utilized in a number of industrial applications. For example, in constructing preforms from which optical fibers may be drawn, vapors of materials such as SiCl.sub.4, GeCl.sub.4, POCl.sub.3 and BBr.sub.3 are generated and delivered to a preform construction site where they are reacted and deposited in or on a support. This is typically done by passing carrier gases such as H.sub.2, He, N.sub.2, O.sub.2 or Ar through a supply of the material in liquid form and to the deposition site with vapors of the material entrained with the carrier gas. In performing this operation a vaporizer is ordinarily used of the type known as a bubbler which has a carrier gas intake conduit that terminates with an outlet orifice located below the surface of the liquid materials, and an outlet conduit extending from the space above the surface of the liquid within the bubbler to the vapor deposition site.
Vapor streams are also generated and utilized in internal combustion engines such as those used in powering motor vehicles. In this application a vapor stream is ordinarily created either by the use of carburetion of fuel injection systems. Where a carburetor is used an airstream is drawn through a carburetor throat into which airstream liquid fuel is drawn by venturi action. From here the airstream is drawn into the intake manifold of the engine along with the fuel in vapor as well as in aerosol and droplet form. With fuel injection type systems electronically controlled injectors are used to inject droplets of the fuel directly into the airstream just upstream of the engine intake manifold.
A long term problem and limitation commonly associated with the just described methods of generating vapor streams has been that of accurately and rapidly controlling the mass flow rate of the vapor present within the vapor stream. For example, to construct an optical fiber waveguide preform properly, the mass flow rate of the vapor must be carefully programmed and accurately controlled. Heretofore, control has typically been achieved with the use of controllers of the type that employs a carrier gas mass flow rate sensor and a vapor to carrier gas ratio sensor. The carrier gas flow rate sensor operates on the theory that the heat added to a known mass of gas is proportional to its temperature rise at relatively constant pressure. They typically employ two resistance heating elements which are part of a bridge circuit positioned in series with each other on the outside of a sensor tube. Gases are passed through the sensor tube which creates a bridging balance, the signal from which is proportional to the mass flow rate. The vapor to carrier gas ratio sensor also operates as a function of heat transfer. This sensor employs one electrical resistance element located in the carrier gas intake stream and another in the vapor and carrier gas stream, hereinafter termed "vapor stream". Again, the sensors are elements of a bridge circuit which indicates any imbalance as soon as the properties of the gas and vapor stream differ. This difference is proportional to the ratio of source to carrier gas.
This just described control system is quite inaccurate with deviations from optimum set points ranging as great as 30% over both long and short terms. This is attributable at least in part to the fact that this method assumes a steady state condition of vapor pressure. In actuality though bubbler type vapor generation systems are not in steady state since mass flow rate depends on numerous criteria such as carrier gas retention time within the liquid, the depth at which the bubbles are released within the liquid, the size of bubbles, total pressure, carrier gas temperature, local temperature inhomogenetities surrounding the bubbles as they travel towards the liquid surface, and heat flow into the bubbler from its environment. These effects all become more important as flow rates increase and the liquid level in the bubbler decreases since retention by carrier gases also decreases as localized cooling takes place.
Vapor delivery rate control is also important in internal combustion type engines. Today, mechanical or electromechanical systems are used in metering combustible fuels into airstreams that are subsequently fed into engine intake manifolds. Being mechanical, their response times are inherently limited. For example, with conventional carburetors an air-fuel mixture ratio is typically controlled by throttling a butterfly type valve in the carburetor throat downstream of the point at which fuel is drawn by venturi action into the airstream. In these systems there is an inherent slowness in response time during the period between which the butterfly valve position is altered and the rate at which fuel is drawn into the airstream is altered. Even where fuel injection systems are used and electronic control enhances response time and mixture regulation, a mechanical device remains the direct operative means employed in actually metering fuel into the airstream.
As a result of these limitations, it is almost inevitable that compromises be made in internal combustion engine design. It would be more efficient to meter varying air-fuel mixture ratios into the engine combustion chambers at different times within the suction phase of the piston stroke. For example, attempts have been made to introduce a relatively lean combustible mixture into each cylinder barrel during the early portion of the suction stroke and a relatively rich mixture during a later portion to enhance burn efficiency with more of a richer mixture being in the vicinity of spark ignition. Heretofore, vapor delivery response times have simply been far too slow to be capable of performing this and little progress made to date at achieving such mixture stratification.
It thus is seen that there remains a need to provide methods and apparatuses for generating vapor streams with enhanced control speed and accuracy in diverse fields of technology. Accordingly, it is to this general problem to which the present invention is primarily directed.